Almost every painting teacher will tell you to establish the big overall statement right away.
The idea is to loosely block in the whole picture immediately, ignoring detail. Scrub some tone over the whole canvas. Later on you can fine-tune the nuances of color and value and add the small touches, like the petals on the rose. This “overall approach” is a sensible way to work, but it’s not the only way to work.
This anonymous, undated, and unfinished figure study was produced by the school of Ingres using an unusual method called "area-by-area" painting.
The artist proceeded in essentially two steps. First, the main lines and landmarks were carefully drawn with a brush. Then the painting was taken directly to finished effect at the first statement, proceeding across the canvas one area at a time.
Here’s a plein-air study of Mount Katahdin by Frederic Church. He worked over a natural-toned paperboard, probably sealed with shellac. He began with an exact drawing of the mountain silhouette. The sky was painted next, very thinly, stippled and blended with the end of a big (probably badger-hair) brush. Then he rendered the delicate construction of the mountain and far shore using semi-opaque oils. He worked from top to bottom, left to right, like a human ink-jet printer.
In this unfinished study, Church captured the rocky coast of Maine in a similar way. On the right, you can see traces of his preliminary—and very precise—pencil drawing.
My suggestion is to follow what your painting teacher says while you’re in the classroom, but next time you’re painting from life on your own time, give this method a try and see if you like the results.
It’s not as hard as it looks. And it’s fast. The trick is to take your time getting the drawing right before you start in with paint, and then mix and render each area with full awareness of its relationship to other values.
What matters is not the method but the result.